Group B streptococci (GBS) are the leading cause of neonatal sepsis, the third most frequent cause of bacterial meningitis and an increasingly important cause of bacteremia and sepsis in adults in the United States today. Preliminary studies have identified a novel proinflammatory component of GBS, which we have designated GBS-factor (GBS-F). Based on experiments in mice with targeted genetic deletions in Toll-like receptor (TLR) expression and on experiments with engineered cell lines, we have determined that responses to GBS-F require expression of CD14, TLRs 2 and 6, and the Toll-adapter protein, MyD88. Activation of this receptor complex by GBS-F initiates important signaling events such as the activation of NF-kB, the phosphorylation of MAP kinases, the formation of proinflammatory cytokines, and the intracellular production of the toxic oxidant peroxynitrate. In contrast, although other components of GBS appear to engage TLRs, the exact identity of contributing TLRs is entirely unknown. The overall goal of this proposal is to identify and define components of GBS, focusing first on GBS-F, and their cognate Toll-like receptors (TLRs). We propose to characterize the structure of GBS-F and assess its function in vitro and in vivo. Furthermore, we will determine if TLR2, and related downstream signal transduction molecules, mediate a variety of important innate immune responses to GBS, including leukocyte chemotaxis and the intracellular killing of bacteria. Finally, we intend to determine what other TLRs, and associated signal transduction molecules, are involved in GBS recognition and response. The data learned from these studies should help in the development of rational therapeutic strategies to interfere with the deleterious hyperinflammation triggered by GBS and similar microbial organisms.